A well-known problem within antenna techniques is how to effectively reduce the radar cross section of an antenna with a minimum effect on antenna performance to thereby enable a stealth performance. One way to circumvent the problem of course is to not use any antennas at all, like for instance the known F117 aircraft. Another possibility is shielding the antenna by means of external frequency selective surfaces (FSS). A third possibility would be to turn away the antenna. However, a turned away antenna can not be used until it is turned back again to the proper direction, which gives an unwanted important negative time factor. Prior art also mentions switched curtains or radomes to be placed in front of an antenna.
A number of documents can be found representing different solutions in connection to the present problem. For instance an U.S. Pat. No. 4,682,176 describes an active transmit/receive module that provides selectable impedance matching between the antenna and the feed lines. The module can be programmed to select a specific impedance when transmitting and another one when receiving.
Another document, U.S. Pat. No. 5,129,099 discloses a tranceiver solution for a radar antenna (array) where one (hybrid mode) phase shifter is used for transmit operations and another for receive operations.
Still another document U.S. Pat. No. 4,117,485 describes a method for reducing, or enhancing, the radar cross section (RCS) by using a load impedance, which may be tuned.
Another recent document WO 02/11239 discloses a multiple band re-configurable reflecting antenna array and a method for multiple band operation and beam steering. An array of dipole antennas is disposed on a multiple band high impedance surface. The antenna array is re-configured by changing the length of the dipole elements, to thereby change the resonant frequency of the dipoles. At a given frequency band, small changes in dipole length allow to steer the reflected beam in a selected direction, while large changes in dipole length result in a switch of operating frequency band.
The U.S. Pat. No. 3,568,194 from 1967 discusses a way in which a return signal can be degraded by using a variable transmission load connected to the reflector. The disclosed method and system comprise phase and/or amplitude modulation of the radar signal by one or more “scattering” sources positioned on a target vehicle, thus causing the return target signal to appear as an incoherent object.
Finally an U.S. Pat. No. 5,153,594 discloses an electronic counter-measure system for installation on an aircraft. The system includes an interferometer transmitter comprising a plurality of repeater amplifier circuits connected in parallel. The plurality of repeater amplifier circuits generate out of phase signals of different amplitudes, which are respectively transmitted by associated spaced transmitting antennas on the aircraft as response to an incoming radar signal.
It is still a demand of further improved solutions to the above-mentioned problem to obtain a feature providing a decreased radar cross section of an antenna without too much effect on antenna performance, to thereby enabling stealth performance.